The use of the sonic and ultrasonic waveforms are typically used to evaluate the quality of the cement sheath between wellbore casing and a formation. A logging tool, which may have one or more sonic and/or ultrasonic receivers and one or more sonic and/or ultrasonic transmitters, is lowered into a wellbore and measurements are taken at various depths. Sonic and/or ultrasonic waves are transmitted from the logging tool towards the formation, and reflected from the casing, cement sheath, and formation. The reflected waves are received, recorded, processed, and interpreted to determine the presence, or lack thereof, of cement between the casing and the formation or other wellbore wall.
Recorded sonic waveforms, commonly referred to as a cement bond log, variable display log or micro seismogram log data, may be plotted with respect to wellbore depth and then visually interpreted by an operator to provide a basic understanding of the composition of the cement sheath in the annular space. Cement bond log data may be plotted with waveform arrival time indicated horizontally, wellbore depth indicated vertically, and waveform amplitude indicated by color or intensity, resulting in a serrated or striped display. Adjacent to this cement bond log plot, a waveform amplitude plot and a gamma ray plot may be provided for correlation.
Waveforms generally have a very different signature when passing through an annular space filled with fluid (free pipe) versus solid (cement). The free pipe signature typically includes higher amplitudes, a low rate of attenuation and a consistent waveform. When the annular space is filled with a solid material such as cement, the amplitude of the waveform is reduced, the attenuation of the same waveform is increased, and the waveforms are not consistent.
In other words, there is typically a significant visible difference in the plotted sonic waveform data between a free pipe response and bonded pipe response. Free pipe is indicated by high waveform amplitude, strong casing arrival, straight or consistent waveforms with depth (indicated by regular striping with depth as plotted), and visible chevron patterns at casing joints. A good cement sheath is indicated by attenuated waveform amplitude, weak casing arrival, and inconsistent waveforms with depth (indicated by irregular striping with depth as plotted).
However, interpretation of a partial cement bond, such as the presence of either a channel, mixed cement and drilling fluids, spacers, or a combination thereof, can be more difficult. There is an apparent transition zone between good and poor cement bonds, and identification of and analysis of the bond in such cases relies on the experience and eyes of the analyst to assess if the cement sheath is adequate for continued wellbore completion operations or if remedial work is required.
In addition to the analysis of sonic waveforms, ultrasonic waveform may be used to evaluate the cement sheath by determining either the impedance or attenuation of the material next to the casing itself. The impedance and attenuation of the material is normally mathematically calculated using properties of the casing, cement, mud and other materials in a complicated method requiring extensive knowledge of a routineer in the field.